Typically, a solid oxide fuel cell (SOFC) employs a solid electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (hereinafter also referred to as MEA). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, generally, predetermined numbers of the electrolyte electrode assemblies and the separators are stacked together to form a fuel cell stack.
Normally, a hydrogen gas produced from hydrocarbon based raw fuel by a reformer is used as a fuel gas supplied to the fuel cell. In the reformer, in general, a reformed gas (fuel gas) is produced, e.g., by applying partial oxidation reforming or steam reforming to such hydrocarbon based raw fuel, e.g., fossil fuel such as metal or LNG.
In this case, since the partial oxidation reformer induces exothermic reaction, the reaction can be started at relatively low temperature and operation can be started efficiently, and the follow up performance is good. In contrast, the steam reformer has good reforming efficiency.
For example, a fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2010-218888 (hereinafter referred to as Conventional Technique 1) is known. In the fuel cell system, as shown in FIG. 4, a fuel processing system 1a is provided. The fuel processing system la has a reformer 2a and a burner combustor 3a. 
In the fuel cell system, an air supply apparatus 5a is controlled based on an indicator value of a flow rate meter 4a. When the air is not supplied by the air supply apparatus 5a, the indicator value of the flow rate meter 4a is corrected to a value indicating that the flow rate is zero. As a result, in the structure, since the indicator value of the flow rate meter 4a indicates the flow rate of the actual supplied air, the flow rate of the air supplied by the air supply apparatus 5a can be regulated with a high degree of accuracy.
Further, in a partial oxidation reformer disclosed in Japanese Laid-Open Patent Re-publication WO 01/047800 (PCT) (hereinafter referred to as Conventional Technique 2), as shown in FIG. 5, a reformer 1b has a dual wall structure including a housing 2b, and partition walls 3b, 3b provided in the housing 2b. A reforming reaction unit 4b is provided between the partition walls 3b, and a space between the housing 2b and the partition wall 3b is used as a raw material gas passage 5b around the reforming reaction unit 4b. 
Heat insulation of the reforming reaction unit 4b is performed by the raw material gas passage 5b to reduce non-uniformity in the temperature inside the reforming reaction unit 4b. The raw material gas in the raw material gas passage 5b is heated beforehand by the reaction heat in the reforming reaction unit 4b. Thus, the heat efficiency in the reformer 1b is improved by self-heat collection, and a preheater for heating the raw material gas beforehand is formed integrally between the raw material gas passage 5b and the reforming reaction unit 4b. 
According to the disclosure, in the structure, in the reforming reaction unit 4b, in the case where a hydrogen rich reforming gas is produced by reaction including partial oxidation from the raw material gas, non-uniformity in the temperature inside the reforming reaction unit 4b is reduced, improvement in the heat efficiency is achieved, and the reformer has a simple and compact structure.